1. Field of the Invention
The present invention relates to a turn signal switch for use as a direction indicator mounted on a steering column of automotive vehicles, and more particularly to a turn signal cancellation mechanism for automatically returning a turn signal switch lever to its neutral position.
2. Description of the Related Art
A turn signal switch on automotive vehicles is rotatably supported at the base end side on a housing which is formed integrally with the steering column, and is so designed as to turn on and off the left or right turn signal lamp by turning the top end of the switch lever in either direction of right or left turn from neutral position. In this turn signal switch, a cam is mounted on the inner surface on the housing side, and an actuator which is engaged with this cam is mounted on the switch lever side through a spring, to thereby hold the switch lever in one of the three positions: right and left turn and neutral positions. Also, there is provided a turn signal cancellation mechanism for automatically canceling the switch lever back to neutral position from either the right or left turn position when the steering wheel is turned in the reverse direction of indication.
As a turn signal cancellation mechanism, there has been adopted a cancel lever, which is rotatably and longitudinally movably supported on a housing. In this canceling mechanism a projection provided on one end of the cancel lever is engaged in a cam hole provided on the switch lever side, and a spring member is mounted to elastically press the other end of the cancel lever towards protruding out of the housing.
In the turn signal switch of rough constitution described above, the switch lever, when in neutral position, is locked in the neutral position with an actuator engaged with the center of the cam provided on the housing. In this case, since the projection is engaged with the crest of the cam hole, the cancel lever is in a retreat position and the forward end of the cancel lever is off the path of rotation of a cancel projection provided on a rotor which rotates in interlock with the steering wheel. In this state, therefore, the cancel projection will not contact the cancel lever if the steering wheel is turned.
In the meantime, when the switch lever is turned from neutral position to the right or to the left, the actuator moves from the center of the cam to either the right or left lock, thereby locking the switch lever in a specific turn signal position and accordingly blinking the right or left turn signal lamp. In this case, the cancel lever projection moves from the crest to the valley portion of the cam hole, being pressed forwardly by the spring member until the forward end of the cancel lever is projected out into the path of rotation of the cancel projection. In this state, when the steering wheel is turned in the reverse direction of the turn signal, the cancel lever contacts the cancel projection and turns during the return operation. Consequently, as the cancel lever projection contacts and pushes the end of the cam hole, the actuator comes off the lock of the cam, thus moving to the steady position at center, and the switch lever turns in the reverse direction of turn signal, being self-cancelled to neutral position.
The aforesaid conventional turn signal switch is constituted so that the switch lever locked in a turn signal position will be automatically returned to neutral position by a driving force from the cancel lever which is turned by the cancel projection. Since a relatively great locking force is set to lock the switch lever in the turn signal position in order that undesired automatic return of the switch lever to neutral position with vibration or other can be prevented, the cancel lever needs a great driving force to cancel the switch lever. On the hand, the cancel lever is operated by the cancel projection which turns in interlock with the steering wheel. Therefore, there exists a limit to the range of angle at which the cancel projection contacts the forward end of the cancel lever. It is, therefore, necessary to largely turn to displace the rear end side of the cancel lever within the limited angle of contact to produce a canceling force.
However, the driving force produced at the cancel lever and the amount of rotational displacement of the cancel lever are in a mutually contradictory relation; for instance, when the support of rotation of the cancel lever is moved close to the rear end side, a great driving force is generated at the rear end side of the cancel lever, while the amount of displacement of the cancel lever becomes smaller. Contrarily to this, when the support of rotation of the cancel lever is brought close to the forward end side, the amount of rotational displacement of the cancel lever increases, while the driving force generated at the rear end side of the cancel lever decreases. The position of support of rotation is set so that the amount of rotational displacement of the cancel lever will increase by slightly sacrificing a decrease in the cancel lever driving force. In this case, it becomes necessary to decrease the lock retaining force of the switch lever with the decrease in the driving force of the cancel lever, resulting in possible malfunction of the switch lever. Where the lock retaining force cannot be reduced, the cancel lever and the cancel projection will be subject to abrasion and breakage, resulting in unsatisfactory canceling function.